In recent years, medical imaging apparatuses (hereinafter referred to as modality apparatuses), which can collect various information about a patient less invasively, have become indispensable in health-care settings. Among others, a magnetic resonance imaging (MRI) apparatus, which involves no radiation exposure and surpasses other modality apparatus in tissue contrast resolution, has come to be used in many medical institutions. The MRI apparatus is an imaging apparatus which excites nuclear spins of a patient placed in a static magnetic field with radio frequency (RF) pulses at Larmor frequency and thereby generates an image by reconstructing a magnetic resonance signal generated from the patient as a result of the excitation. To acquire a high-contrast image with an MRI apparatus, it is necessary to tilt the nuclear spins of the patient at a desired angle by application of the radio frequency pulses. The tilt is referred to as a flip angle and magnitude of the radio frequency pulse is expressed by the flip angle. That is, to acquire a high-contrast image, accurate radio frequency pulses need to be outputted from the MRI apparatus.
The radio frequency pulses applied by the MRI apparatus is used as energy to give a tilt to the nuclear spins and other part is used as thermal energy to heat the patient and raise temperature of the patient. Thus, in the use of the MRI apparatus, from the standpoint of safety, a specific absorption ratio (SAR) has been defined as energy absorbed per unit mass of the patient and an upper limit of SAR, i.e., a safety standard value of SAR, has been prescribed as an IEC (International Electrotechnical Commission) standard (IEC 60601-2-33). More specifically, SAR (unit: W/kg) is defined as energy of the radio frequency pulses absorbed by 1 kg of living tissue, and upper limits of average SAR over arbitrary 10 seconds and average SAR over the most recent 6 minutes have been prescribed for each imaging site such as the whole body or the head. In order to carry out imaging such that the SAR will satisfy the safety standard value, the radio frequency pulses applied to the patient have to be accurate.
Thus, an MRI apparatus is provided which predicts an SAR value based on imaging conditions and optimizes a sequence of imaging protocols so as to optimize the safety standard value.
However, to carry out SAR-based safety management strictly, it is necessary to calculate the SAR value accurately. Thus, an MRI apparatus is provided which accurately calculates the SAR by directly measuring an electric current flowing through a transmitter coil by means of a scan performed prior to an examination of the patient and known as a prescan and calculating electric power used on the patient, based on the measured electric current. Furthermore, an MRI apparatus is provided which calculates an amount of power consumption from coefficients of a region, bed position, or the like by taking into consideration an amount of loss of the radio frequency pulses actually emitted to the patient, predicts the SAR value based on the calculated value, and thereby modifies imaging conditions.
In this way, techniques are provided which accurately calculate SAR values by calculating the energy emitted to the patient.
The techniques described above can calculate more accurate SAR values and strictly carry out SAR-based safety management. However, it is not known whether or not the radio frequency pulses are outputted as set out in imaging conditions, and if radio frequency pulses actually outputted are much stronger than the setting, the SAR value increases greatly, which could have resulted in a need to change the set imaging conditions. Also, even if the SAR value is measured accurately, there is a problem in that desired image contrast is not available if the radio frequency pulses are not applied at set power.
Radio frequency pulses are outputted in a pulse manner multiple times in one examination and the outputted radio frequency pulses fluctuate in real time due to heating, load changes, aging degradation, and the like of elements used in an amplifier and transmitter coil and the like of a radio frequency pulse transmission circuit. That is, actual output could deviate in real time from radio frequency pulse output expected at a time of imaging condition setting. Such deviation could cause imaging to be performed at an output higher than predicted SAR value, requiring time and effort to change the set imaging conditions, or could cause imaging to be performed at an output lower than set radio frequency pulses, making it impossible to obtain desired image contrast. Consequently, in an examination using an MRI apparatus, it is necessary that the SAR value is measured correctly and that the radio frequency pulses are applied to the patient at an output as set out in imaging conditions.
Thus, there is a demand for an MRI apparatus which can apply more accurate radio frequency pulses to the patient.